Il8 GEORGE DELWIN ALLEX. 



"indefinite" in weaker current, the stronger current usually 

 called forth the positive reaction. 



IV. REVERSAL OF REACTION* INDUCED BY CHEMICAL 



CHANGES. 



Reversal of reaction from positive to negative can usually be 

 induced by pouring off the water from the worms and replacing 

 it with fresh tap-water. Thus in 23 out of 24 observations 

 this treatment resulted in reversal to the negative reaction. Two 

 factors may be responsible for this reversal, change in temperature 

 and change in the chemical composition of the water. Each 

 of these factors was investigated by modifying the condition in 

 question while preserving other conditions unmodified. The 

 velocity of the current could not be preserved unmodified with 

 the methods employed, but this factor was controlled indirectly. 



The effect of changing the chemical character of the water 

 was tested as follows: A lot of worms were tested in the water 

 in which they were living at the time. Then an equal volume 

 of fresh tap-water, or modified water, was brought to the same 

 temperature as the aquarium water, and the worms were tested 

 alternately in these two kinds of water. 



A number of experiments were performed in which aquarium 

 water was replaced by fresh tap-water of the same temperature. 

 Since the rheotropic reaction in many cases may be positive in 

 stronger current and negative in weaker current, it must be 

 made certain that the reversal of reaction after a chemical change 

 in the environment is a reversal of reaction in the same velocity 

 of current. Although it seemed fairly certain to the operator, 

 during the earlier experiments, that there was such a reversal in 

 the same velocities of current, the later experiments were per- 

 formed with a "control" to make them more conclusive. In 

 these experiments two dishes of worms living under the same 

 conditions, frequently in the same aquarium tank, were tected 

 side by side. One lot of worms was tested in waters of different 

 composition while the other lot was kept as the "control" and 

 was tested always in the original aquarium water. The ex- 

 perimental and control dishes were placed side by side, and the 

 control worms were tested at each trial simultaneously with 



